This invention relates to electrical power assisted steering systems of the kind in which an electrical motor is adapted to apply an assistance torque to a steering component such as a steering column so as to reduce the driver effort required to control the vehicle.
In a simple electric power assisted steering system a torque sensor is provided which is arranged so that the level of torque in a steering column is measured. From this measurement a controller calculates the value of a torque demand signal which includes an assistance torque component that is indicative of the torque that is to be generated by an electric motor attached to the steering column. The motor applies an assistance torque to the column of the same sense as that demanded by the driver and thus reduces the effort needed to turn the wheel.
A problem with this simple arrangement occurs in certain driving manoeuvres which excite a vehicle yaw mode transient response—leading to so-called “fish-tailing” of the vehicle. These manoeuvres are typically the result of “unsupported” driver actions on the handwheel such as rotational “flicks” where the driver applies a rapid handwheel angle change but does not follow it through with any substantial applied torque or perhaps releases the handwheel after initiating a rapid turn.
In such circumstances it is desirable that the handwheel returns to the central “straight-ahead” position quickly and with a minimum amount of overshoot or oscillation. In general, however, geometric and inertial effects of the steering system contribute to a free mode yaw response that is lightly damped and quite oscillatory—particularly at high vehicle speeds.
It is known in the art to overcome this problem by including a damping component within the torque demand signal that is used to drive the motor. This damping component in some sense mimics the mechanical phenomenon of viscous friction through software by generating a component of torque demand that is a function of the handwheel velocity. The damping component generally increases in magnitude as a function of steering angular velocity from zero torque at zero rotational speed to a maximum at some arbitrary maximum speed. In effect, the damping component reduces the actual torque output by the motor, and hence the amount of assistance, in a particular instance when the velocities are high. This gives increased damping and hence stability at high vehicle speeds.
It is further known to provide an electric power assisted steering system in which the damping component is a function of the torque carried by as well as angular velocity of the steering column with the damping component being reduced at low torques compared to the magnitude of the damping component at high torques. Thus, in hands free manoeuvres where no torque is present in the column the damping will be relatively high and yet be lower during hands on manoeuvres in which torque is generally present in the column.
The reduction of damping at low torques in this way will always be a compromise between the requirements of damping during hands on manoeuvres and hands off manoeuvres. In order to minimise the intrusion of damping during hands on manoeuvres it has been proposed to make the threshold at which the damping switches from a high value to a low value very close to zero torque. This has been found to minimise the intrusion of the damping during hands on manoeuvres.
The use of a very low switching threshold value, whilst generally presenting a good steering feel, can produce some undesired effects during a swerve or rapid lane change in which the driver is holding on to the wheel (hands on) and rapidly rotates the wheel first one way and then back in the other direction. In this type of manoeuvre the driver will be demanding rapid oscillatory changes in the vehicle direction over time, and the steering wheel velocity and the torque applied to the wheel will conform to an approximately sinusoidal pattern. During a swerve there will be moments when the driver applied torque and the velocity will peak and also when the torque passes though zero torque on at least one occasion. The low threshold will ensure that no undesired damping is present at all times in this type of manoeuvre except for a band around the zero torque crossing in which case the damping will suddenly increase as the torque approaches zero and then decrease after the zero crossing. In effect, the system is confused into thinking that the point around the zero torque crossing corresponds to a hands off situation in which damping is needed. This is illustrated in FIGS. 7(a) and 7(b) which are plots of column position and damping torque over time during a simulated series of swerves of a small passenger type car. Feeding a frequency swept sinusoidal input into a model of the system simulated the swerving.